Control signal indicating circuit



May 29, 1951 J. w. SMlTH 2,554,508

CONTROL SIGNAL INDICATING CIRCUIT Filed Dec. 5, 1948 MmlHw- FACSIMILE ITRANSMITTE AMPLIFIER BALANCED MODULATOR AND PRE-AMPLIFIER INVENTOR.

JOHN W. SMITH A TTORNE'Y Patented May 29, 1951 CONTROL SIGNAL INDICATING CIRCUIT John W. Smith, Whitestone, N. Y., assignor to Faximile, Inc.,New York, N. Y., a corporation of Delaware Application December 3, 1948,Serial No. 63,314 3 Claims. (Cl. 332-39) The present invention concernsfacsimile systems and, in particular, facsimile transmitter levelcontrol indicating systems.

In a facsimile system, electrical signals are generated representinggraphic and textual materials to be reproduced. The amplitude of thesignals represents the variations in density of the subject copy fromminimum to maximum. There may be considerable variations in theamplitude of the signals due to great differences in density range ofthe subject copy. Copy with a gray or darkened background, for instance,may have a much smaller density range than sharp black copy against apure white background. Many systems pick up the light variations fromthe sub- .ject copy in the scanning process in a photoelectric cell andutilize the initial signals generated in the photoelectric cell tomodulate a subcarrier in a balanced modulator. One end of the densityscale, as represented by the electrical signals, may be set by the pointat which the balanced condition is obtained in the balanced modulator.This balance may be obtained for either maximum or minimum density ofthe subject copy, although, in many ways, it seems desirable to set thebalance at the minimum copy density point.

This is sometimes called white balance. With one end of the densityscale determined by the balance point, it is also desirable to definethe limits of the excursion of the electrical signals representing thedensity variations, in order to utilize most efiiciently and effectivelythe characteristics of the facsimile transmission and recording systems.Thus, a simple and accurate method of determining the modulator balancepoint and the maximum desired density of signal excursion is a desirablefeature in a facsimile transmitting system.

The present invention concerns a simple, yet accurate, method and meansfor indicating the setting of the balance point and the maximum signalexcursion in a facsimile transmitting system. The indicating device inthe system consists in a novel circuit arrangement for the use of anelectron ray indicating tube. With a facsimile scanner set on a portionor" the copy at which the balance is to be determined, signals areamplified, rectified and utilized to actuate the electron ray indicatingtube. The balance setting of the modulator is then manipulated until nosignal is indicated as being received by the electron ray tube and,therefore, balance is obtained in the modulator. By utilizingamplification before the rectifier feeding the electron ray tube,

this system may be made as sensitive and accurate as may be required inany given case. In order to set the maximum signal excursion, a variablebias is applied to the rectifier and adjusted to substantially equal thedesired maximum signal excursion. Then, the transmitter amplifier gaincontrol is adjusted until the peak of the maximum signal just equalsthis bias as indicated by its effect on the electron ray tube.

One object of the present invention is to provide a simple method andmeans for determining the minimum signal and the maximum signalexcursions in a facsimile transmission system, or the like.

Another object of the present invention is to provide a simple andaccurate device for determining when a small signal voltage exists in afacsimile system or the like.

Still another object of the present invention is to provide a simple andaccurate device for determining the maximum signal amplitude in afacsimile system or the like.

A still further object of the present invention is to provide means forutilizing the same indicator for indicating minimum signal in afacsimile system or the like, and, by means of a switch arrangement andadjustable bias, to indicate a predetermined maximum signal excursion.

These and other objects of the present invention Will be apparent tothose skilled in the art from the detailed description of the inventiongiven in connection with the drawing.

The figure shows a preferred form of the present invention in connectionwith a facsimile transmitter.

In the drawing a facsimile transmitting drum I,

, carrying the subject copy 4 is rotated by a motor 2, through shaft 3.Scanning box 5 generates electrical signals representing the subjectcopy density variations in any convenient and suitable manner andsignals thus generated are applied over leads 6 and l to balancedmodulator 8. The signals received by balanced modulator 8 are utilizedto modulate a suitable subcarrier and are amplified by a preamplifierand set to an output level control consisting of potentiometer 9connected between balanced modulator 8 and ground G. A portion of thesignals appearing across potentiometer 9 are applied to facsimiletransmitter [3 in an amount determined by the setting of contact armii). The signals reaching transmitter :3 are utilized in any convenientmanner to modulate the facsimile transmitter and are radiated fromantenna M and ground G to be recorded at some distant point.

At least a portion of the signals applied to transmitter l3 are alsoapplied to the indicating device which is the subject of the presentinvention. These signals are applied to amplifier l6 through seriesisolating resistor l5 and across shunt resistor I! by means of lead I8.After suitable amplification, the signals are applied across resistor26, through coupling capacitor IS. The signals across resistor 20 areapplied to diode 2] which includes cathode 23, heated by suitable meansnot shown, and plate 22. A switch.

r' seen-"thatyby use of the apparatus-described in including movablecontact 36 and stationary conthe manner outlined, very accurate settingsof tacts 38, 39, 31 and 40, is used to switch cathode balance conditionand maximum desired signal 23 eithenltcggroundlG or to contacts-arm 35on streng'th in the balancedz mgdulator niambe easily potentiometer"34.When contact 36-is depressed and-quickly obtained. *By the use-of anelectron to short contacts 3B39, cathode 23 is connected 5 rayindicating device, the problems of inertia to ground G and all signalsappearing across re- "present in the normal meter type circuits aresistor 20 are rectified by diodezii';'generating:=:a -overcome andan,inexpensive, but highly sensirectified signal which makes plate 22negative tive and accurate indicating system is obtained, with respectto ground G. Thenegativebias thus -whichwillimmediately determine theproper siggenerated is filtered through"resistor""2 i,'-across '10-nal-strength-settings for minimum signal condioapacitor 25 and isapplied to control grid 29 of tion or'fu'll'signalcondition for any typeof copy the amplifier section of thei'electronxray-ltubeilfi.gmateriahbe ll nsed. Electron ray tube 26, which may be oilthe typeThough only one means of accomplishing the designated as 6E5, includescathode 28; control "resultobtained has been shown, it will be evidentgrid 29, amplifier plate fill, and, connected to plate to those skilledin the art that other types of cir- 'Sfi, control elec-trode 33;electronray cathode 21, cuits-may be employed .to..produce.similarsresandsele'ctron'ray'plate 32. Plate'32is connected -within'the inventionas .defined; in; the .following to a voltage--source suchas-battery toand plate claims. Bilisconnected through a-loadresistor 3-! to volt-Whatp-is 'claimedis: age source' lo. *Potentiometer3-isconnected-be-1 1. :In-adeviceof z-tthe class rdescribed t'thecc omtween voltagesourceflit and ground G- so that its b inationpfsource of signalstogbemonitored, contactor'fiinay-bevaried from a point at-ground means foramplifying at .least; a portion; of the potential up to a point equal totlie potential of signals;: means'.for.rectifyingsaid amplified sig-':battery--4G. --When*-the rectified signalsabovedenalsto proyidecontrol signals, anelec n; ray

scribed are filtered throughresistorand -ap- 25 indicating device; meansfor applyingsaid plied-to' grid 29; they will cause the eye of thesignals-to-saidelectron ray device to indie electron ray tube to close.The amount by which :presence of signals from. said sonrce,.,a-, swit,:itcloses dependsupon theamplitude of there- .iionplacing abias onlsaidrectifientopreventinceived signal. Inorder'to set the balanced-mod--dication-oi the control signalsmntil;saidc "ulator*8"accurately at-abalance' point,'scanner-. "piified signals reach.alpredeterminednleyel and pick-up 5 is set to pickup an area on copy 3 "means foradjustingsaidbiastota;predeterm ed *Which has a density equal'to thedensity at -which o ating point to;determinecthe.; signalfl ylz''-theb'a1ance-is made. *Thus ii the balance is to --w ich said electronray .tubeindicates. "be made for-white, the-normal type of balancei2lnl-a factri'milei f fi l l' th mmb n condition; a -portion ofcopyiwhich-is-white ish ztiOn 1;.of,na: a1 nced; mil sll fi i 1 le q layplaced under the scanning-beam from scanner 5.'indicating.rtubeiiorcindicating the Jbalance -con- The balancedmodulator balancing control-,tvhich 46113110111 40f sai cbal nce m du man i may be-of any suitable construction, not shown, is icluding v -b ac ource; anda switch v o c ye theirmanipulateduntil the electron eyeopens zingssa r y tuoeto. ape kq r a inam- *fu'lly," showing that novoltage is being received fl monitoring :themaximum i 1.a DrO- acrossresistor 2c. The-sensitivityand accurac tvidedbysaidbalanced modulator.of this adjustment may be carried to any desired J 3. Inca;(19371631110? monitorin th op rat n degreeby suitable amplification inamplifierl6 --c0nditionsoi atbalanced modulator, th cqmbin and istremendously increased by *the lack of QDfi J bemon ore -i *inertia'izrthe eye indicator. -With the'balll5 ingcaibalanoed m0dulator,: an,6=t11 ay ance point setfthe-drum d is rotated and a-point ndic'atortube, a circuit connecti said. indiicai fl of maximum density'placedunder-the scanning :tube to said .sourcerofs ignalsa: rectifier;conbeam-so that themaximumsignal-strength'fornectedin;:shuntwitlusaid:.circuit; f0 ectif ing the copy-is generatedand appears-across potenwslgllals rom saidtsouice iof signalscand J01ap- "tiometer-t. Contact '36 is-moved to its uppe1-5o-p y re tifie nal:vol a elto said indicator position, shorting contact Eli and placing aw fl "l'fiference.voltage, andtaptt "bias on-cathode tiiearnountof thebe- 'p nwi h n in one si aofr aid recingdetermined by the position ofcontact-65011 tifier to a substantially neutralreference voltagepotentiometer :54, 4 This bi s prevents any c r- :-p0int.in..'itsfirst-position; andto said reference rent flow in diode 2! until thereceived signal 'Vo1tageLs0urce in'.aits ,sec.ondi pos onQfOT-attappearing across resistor ae-has-pea-kswhicn eX- -curately determin nm dulator balance in:-the ceed the bias. From previous calibrations ofthe *"fllslllswitch -P and. o ng l h system, slider-35 is usuallysemipermanently ad- 'imaximllmfsignalilevel-in the secondSwitchposijusted so that the proper value maximum signals appearingacross resistor'2i! and rectified by di- JOI-INWsSMITH. ode 2| generateanegative bias on control grid as suiiicient to cause the electron eyeto just close. kTREFERENGEs-CITED If y does not @1036, if it 81056569 sThe following'references are of record --in"the overlatgii sliderglfifonpotentiometer 9- is adjusted file of this patent: I unti e poin ojust-closing is reached. Ihis adjustment, as determined by previouscalibra- (M "UNITED STATES PATENTS tionsgis then such'that, facsimiletransmitter 13 Number 1 Name 5 3 11 receives exactly the rightpeaksignals during the l F ch V J an. 26, 1937 scanning process ofcopy'4.'If the copy 4 is 3 5 PJOnker P 1 ,c the prccessis repeated t t is,thebab v{231:595293 :Shonnard .Jan. 13,519.49

-ancein the balanced modulator 3 is reset'a-nd the OTHER REFERENCES"maximumsignal"level is also reset by' readjustment of slider l G onpotentiometer 9. v It isreadily f 1R V' W for Jamary 1937ppages 1 19 120,

QST .for :November 1936; ;pages; 23 26.

